Mary Riddell is a columnist and a political interviewer for the Daily Telegraph. She writes on topics ranging from family to foreign policy and is particularly interested in criminal justice. Her focus is what is going on, for better or for worse, in the Parliamentary Labour Party.

The leaders' debate could be the beginning of the end for David Cameron

Tory strategists say that winning this election is like carrying a Ming vase across a room. One stumble, and the treasure is smashed. So how bad was the leaders' debate for David Cameron? In many ways, he didn't do much wrong. Implying we might be nuked by China may be an egregious lapse, but most voters won't mind much. Nor may they be bothered by the thud of clichés ("We're all in this together") or even the lack of clarity on how to pay off the deficit. But though he stayed calm, Mr Cameron's face seemed to register a growing horror. The Tory nighmare had been that he might lose out by appearing a master of presentation. If only.

Nick Clegg 'out-Daved' David Cameron (Photo: Getty)

Nick Clegg did not exactly pirate the Cameron playbook. Instead he produced a more novel and convincing version. You could see from Mr Cameron's eyes that he knew he had been out-Daved, both on image and policy. Mr Clegg's line on Trident and his emphasis on stopping children becoming career criminals both belonged in the no-go terrain where ordinary politicians dare not stray.

Yesterday Mr Cameron looked ordinary. Gordon Brown also failed to dazzle, but no one – least of all his strategists – ever thought he was going to. He did not scowl or gurn, he was solid on substance and the economy, and he even managed, albeit wincingly, to spit out the name "David". Expectations were set low for Mr Brown, and he slightly exceeded them.

The Lib Dems are celebrating today. Their polling is telling them that, even before the debate, they were making headway against the Tories in the South-west. Now they have their hopes set on the cathedral towns and beltway marginals where they could oust Labour. Much as Gordon Brown might cast lovesick glances towards a LibLab deal, Mr Clegg is going to play hard (and maybe improssible) to get.

But a relatively small shift towards the Lib Dems will help Labour anyway. And, should there be a hung Parliament, it seems unlikely, if not unthinkable, that Mr Clegg would ever do a deal with Mr Cameron. Much as a handful of Lib Dem MPs would like it, policy differences and the LibDem party membership would almost certainly forbid such an accord.

No wonder Lord Mandelson and Alan Johnson were smiling. Brown got it on substance and Clegg on style. This, they hope, could signify a match made in heaven. We shall see. There are many potential slips between now and polling day, but the Tories' unsealed deal may have started to unravel.

Last night could prove to be the moment when the Ming vase hit the floor.